1. Field of the Invention
This invention relates to the fabrication of complex microcircuit boards and substrates, complex semiconductor circuits using the microcircuit boards and substrates and methods of producing same.
2. Description of the Prior Art
The art of forming shaped articles from particulate mixtures is well known. Classically a desired particulate material is mixed with a binder and then formed into the desired shape, this being called a green body. The green body is then fired to provide a fusion of the particulate material and drive off the binder, thereby producing the desired shape product with proper surface texture, strength, etc.
In the production of micro-electronic substrates in the manner above described, it has been a practice to mix the particulate material, usually ceramic, and a thermoplastic binder system together and cast the mixed system onto a moving belt. The cast mixture is then adjusted to a given thickness by the use of a scraper or doctor blade. The green ceramic material, thus doctor bladed to a given desired thickness, is usually permitted to evaporate out a portion of the thermoplastic binder and the green dried sheet is then cut into desired lengths for further processing.
The green ceramic sheets thus formed have had a considerable amount of effort and cost expended in maintaining them to a specific thickness and every effort is also expended to maintain the sheets throughout their processing in a configuration in which the surfaces are as flat and parallel as possible. In subsequent manufacturing steps, alterations are made to this thin, flat, parallel-faced geometry by such techniques as punching holes through the green ceramic, laser scribing the green ceramic to cause depressions in the green ceramic, the passage of the green ceramic beneath a saw or grinding wheel to scribe or machine a configuration into its surface and other techniques that are very well and thoroughly described in the literature of this art.
Subsequently, additional coatings are applied to the ceramic by one of several well known techniques, such as silk screen printing on to the flat surfaces of the ceramic substrate. In any case, the accepted practice starts with a ceramic substrate which is manufactured in a controlled flat configuration and is subsequently worked by a machining or other mechanical manufacturing process to alter the surface configuration to something other than a flat surface. As the green ceramic is quite delicate, the degree to which the flat, as formed, surface can be mechanically altered into a complex three dimensional configuration is severly limited and the ability to closely control the geometry of the holes is also severely limited.
In a complex hybrid substrate in which a great many components, both active and passive, are located on the substrate and interconnected to each other and to the outside world, the interconnection problem is extremely severe. Many components that are affixed to the substrate, such as micro-electronic circuits and transistors, are interconnected with the conductive paths formed on the substrate by means of fine wires which are welded to sensitive areas, termed pads, on the transistors or micro-electronic circuits and terminated on the substrate by another weldment to a conductive area on the surface of the substrate. The wires that are utilized for these complex interconnections are extremely fine on the order of magnitude of 0.0254 millimeters in diameter. Frequently due to the interconnection requirements of the particular hybrid circuit, the wire must pass over another conductive path usually, though not necessarily, limited to a metallized region on the substrate. When this is necessary, what is usually done is to provide,through multiple silk screening and firing techniques, an insulating layer that separates the two conducting paths so that a short circuit will not occur at the region of cross over.
Another problem prevalent in conventional substrate manufacture is that when the prepared substrate is fired, the shrinkage due to sintering is not uniform and not totally predictable. When the non-uniformity of the shrinkage of the substrate exceeds the tolerance limits of the subsequent silk screen printing operations on the surface of the substrate, it becomes impossible to register the printing on the substrate with any through holes that may exist on the substrate. This effect places a limitation on the size of the circuit being fabricated as any arbitrarily small size must be within the limits of these two independent manufacturing steps.
Another limitation found in conventional high density microcircuit boards and substrates is the dissipation of the heat generated in the high density packaged components in that the heat generated by placing components closer and closer together becomes progressively more difficult to dissipate and get away from the surface as the circuit density increases. The conventional solution to this problem is to employ a substrate material that has the highest possible thermal conductivity. This is generally beryllium oxide which is generally considered to be highly toxic.
A yet further problem inherent in prior art multiple layer ceramic substrates is the fact that the green substrate surfaces are not flat after the thick film material has been applied to that surface because the thick film material provides a ridge above the surface and, for that reason, when the "green" substrates are layered, one atop the other with conductive patterns therebetween and pressure is applied to the multilayer "green" substrate, excessive stresses are set up that result in low fired yields of the sintered multilayer substrate.
A still further problem is that, in thick film devices, as the conductor dimensions become progressively smaller, the cross-section geometry of the applied conductor layer becomes less predictable as it is applied as a silk screen ink in thick film technology. Thus, the distributed circuit constants such as inductance and capacitance have a wide range of unpredictable values from substrate to substrate and, in circuits that are critical to these parameters, they must be individually trimmed after manufacture. If the thin conductor is buried in a multiple layer configuration, the post sintering trimming becomes impossible.